Project 2 Abstract Project 2 will generate structural information to validate and improve the design of new generations of SOSIP trimers in close interaction with Project 1 and Core B. Project 2 will take a highly integrative approach involving cryo- and negative-stain-electron microscopy (NS-EM), X-ray crystallography and a range of other biophysical tools (SEC-MALS, ITC, DSC and SPR) to provide a comprehensive biophysical understanding of trimers and trimer-bnAb complexes. The data will be shared with Project 1 to improve the stability, antigenicity and immunogenicity of new SOSIP trimer designs. Dr. Ian A. Wilson will direct Project 2 at The Scripps Research Institute and Dr. Andrew B. Ward will be co-Leader. The Specific Aims are: Aim 1: To biophysically and structurally characterize new SOSIP trimers, including germline targeting SOSIP trimers. We will generate high-resolution structures by cryo-EM or X-ray crystallography to enable further engineering SOSIP trimers, and low-resolution structures by NS-EM to provide rapid feedback about the quality of SOSIP trimers. We will also use NS-EM to assess the quality of trimers after stress tests that mimic vaccine storage and formulation conditions such as the effect of adjuvant mixing. These assays are critical prior to animal immunization studies, to ensure only high-quality trimers are evaluated. Aim 2: To determine high-resolution structures of SHIV infection-elicited, vaccine-induced NAbs, or nave precursor antibodies isolated with GT-SOSIP trimers. We will solve structures of neutralizing Abs (NAbs) isolated from trimer-immunized and SHIV-infected animals to assess their similarity to known bNAbs and their potential for developing breadth. The goals are to further optimize the presentation of bNAb epitopes on SOSIP trimers and facilitate the design of sets or series of trimers for steering responses toward bnAbs in prime-boost-boost strategies that mimic infection. We will solve structures of SOSIP trimer complexes with germline precursor antibodies to compare with affinity-matured bnAbs. The goal is to design SOSIP trimer boosts for guiding precursor antibodies toward breadth. Aim 3: To design, optimize and determine structures of SOSIP trimers displayed on nanoparticle platforms. We will incorporate SOSIP trimers into various proteinaceous nanoparticles (NPs), such as the two-component self-assembling NPs, via an iterative cycle of structure-based design and testing. The goal is to preserve the desirable antigenic properties of SOSIP trimers, while gaining the immunological benefits of particulate antigen presentation. Working with Project 1 and Core B, we will evaluate a range of designs and thereby generate an arsenal of SOSIP-NP immunogens that will be tested in animals. The outcomes of those studies will further guide the design of new SOSIP-NP constructs.